This invention relates generally to an impact printer and, in particular, to an impact printer in which printing is effected by striking the tips of printing wires against an ink ribbon.
Conventional impact printers commonly include printing wires disposed in wire guides and driven by electric coils, for impacting an ink ribbon against recording paper. These printers have drawbacks because ink from the ink ribbon enters a gap existing between the printing wires and the wire guides. This ink interferes with the motion of the printing wires which can result in omitted dots or uneven printing darkness.
A lubricant is commonly included in the print head of conventional impact printers to improve the durability of the elements within the head. However, the lubricant can hinder the movement of the printing wires or levers that drive the printing wires. This also results in omitted ink dots and uneven printing darkness.
To overcome these problems, conventional printers commonly include a warm-up operation before printing begins. The warm-up is typically controlled by a combination of (1) a timer which outputs a warm-up signal when it detects that a printing signal has not been received for a predetermined period of time and (2) a pulse generator responsive to the warm-up signal for driving the printing wires, but to a small extent so that printing does not occur. This warm-up operation is also described in Japanese Post-Examination Publication Number 58-45351 (1983).
This prior art warm-up method suffers from the following drawbacks. The warm-up process only occurs when it is detected that printing was not carried out for a predetermined period of time. However, it does not take the temperature of the ink and lubricant into consideration. Temperature has a substantial effect on the ability of ink from the ink ribbon and the lubricant to interfere with movement of the printing wires and the levers driving the printing wires. Accordingly, unnecessary warming-up is frequently carried out which decreases the throughput of the printer.
Other conventional warm-up methods that depend on temperature have drawbacks. If the warm-up printing wire driving conditions are set so that warming-up occurs when the temperature is low and the wires move without causing printing to occur, the output of the force of the printing wires typically becomes too strong when the temperature is ordinary or high. This can result in staining the printing paper. If warming-up driving conditions are set so that the printing paper will not be stained, the effectiveness of the warming-up operations is diminished at low temperatures.
Conventional impact printers therefore have inadequacies due to these shortcomings. Accordingly, it is desirable to provide an impact printer which avoids the shortcomings of the prior art and prints clear uniform characters without skipping data, improperly varying the print darkness or staining the paper as well as avoiding excessive down time.